The present invention relates to a cushion ring that is used between a rod-side cylinder head and a piston rod of a fluid-pressure cylinder and is externally fitted on the piston rod to cushion a shock in the vicinity of the farthest extended end of cylinder, and a fluid-pressure cylinder provided with this cushion ring.
Of fluid-pressure cylinders using an oil pressure, a water pressure, and the like, a fluid-pressure cylinder in which a working fluid is supplied to both the bottom side and the rod side of piston to generate a linear driving force uses a cushion ring to cushion a shock produced when the working fluid is supplied to the bottom side and thereby a piston rod extends and the piston comes into contact with a rod-side cylinder head, that is, a shock in the vicinity of the farthest extended end of cylinder.
One example of the fluid-pressure cylinder using such a cushion ring is explained with reference to FIGS. 3a-3c. FIG. 3a is a longitudinal sectional view typically showing the fluid-pressure cylinder using a cushion ring that is the related art of the present invention, FIG. 3b is an explanatory view for explaining the operation of the cushion ring in the case where a throttling groove of the cushion ring shown in FIG. 3a has a V-shaped cross section, and FIG. 3c is a sectional view taken along the line R-R of FIG. 3b. 
A fluid-pressure cylinder 30 shown in FIG. 3a, which is described in Japanese Unexamined Patent Application Publication No. 2002-31106, includes a piston rod 22 externally fitted with a cushion ring 21, a piston 23 provided in the base end part of the piston rod 22, a cylinder body 24 for sliding the piston 23 therein, and a rod-side cylinder head 25 that seals the rod side of the cylinder body 24.
The cushion ring 21 is externally fitted in a reduced-diameter part of the piston rod 22. To this reduced-diameter part, the piston 23 is further fixed in an externally fitted manner. The cushion ring 21 slides together with the piston 23 and the piston rod 22 while being restrained by the piston 23.
At the outer periphery of the cushion ring 21, a slit 21a extending from an appropriate position in the axial length of the cushion ring 21 to a rod-side end part is provided, so that the cushion ring 21 cushions a shock in the vicinity of the farthest extended end of cylinder by throttling the flow quantity of working fluid flowing from a rod-side fluid chamber A to an input/output port 25a provided in the rod-side cylinder head 25.
In relation to this throttling operation, on the inside diameter side of the rod-side cylinder head 25, on which the cushion ring 21 is fitted in, a spacer 26a, a cushion seal 26b, and a collar 26c are provided in the named order from the bottom side.
For the fluid-pressure cylinder 30, the spacer 26a can move in the axial direction, and the inside diameter of the spacer 26a is larger than the inside diameter of the cushion seal 26b, by which vibrations of the cushion seal 26b are suppressed, and thereby noise is prevented from being produced by the vibrations (paragraph [0031] of Japanese Unexamined Patent Application Publication No. 2002-31106).
On the other hand, the slit 21a at the outer periphery of the cushion ring 21 functions as a throttling groove for working fluid to cushion a shock. Depending on the application, this groove may be made a throttling groove 21a′ having a V-shaped cross section as in a cushion ring 21′ shown in FIGS. 3b and 3c. 
If such a throttling groove 21a′ having a V-shaped cross section is provided, as shown in FIG. 3b, when the cushion ring 21′ enters into the rod-side cylinder head 25 in the vicinity of the farthest extended end of the cylinder, a jet stream throttled by the throttling groove 21a′ collides with a U-packing 26d′ on the rod side, so that the U-packing 26d′ may be damaged. Therefore, improvement has been desired.